Research paperBinuclear zinc(II) complexes of N(4)-substituted bis(thiosemicarbazone) ligands incorporating hydroxyl group and their non-hydroxyl analogues
Graphical abstract
Using structural modification on the ligand, we were able to synthesize four zinc(II) complex with two different geometric structure. The interaction of the zinc(II) acetate salt with N(4)-substituted bis(thiosemicarbazone) ligands involving hanging hydroxyl group and their non-hydroxyl analogues created binuclear complexes with ligand to metal molar ratios of 1:2 and 2:2 (dimer), respectively. Fluorescence studies showed that the fluorescence intensity depends on the substituent on the terminal nitrogens and geometric structure of the resulting complexes, as the strongest fluorescence was found for [ZnL1]2·MeOH complex with a dimer nature.
Introduction
The zinc complexes of thiosemicarbazone ligands are excellent alternatives for fluorescent imaging agents [1], [2], [3], [4], [5], [6]. Zinc complexes not only are readily available, but also they are generally less cytotoxic to cells and are conveniently monitored as well [7], [8], [9]. The versatile coordinating behavior of bis(thiosemicarbazone) ligands and the structural flexibility of zinc(II) cations leads to the formation of a variety of zinc bis(thiosemicarbazone) complexes with different coordination numbers and geometries [10], [11], [12], [13], [14], [15], [16]. Structural data for the [MN2S2] complex unit resulting from the coordination of bis(thiosemicarbazones) with copper(II) and nickel(II) are well-known [17], [18], [19]. We also discussed the review of such complexes in two our previous work. Both ions preferably form mononuclear planar complexes [20], [21]. Since Zn(II) usually favors tetrahedral coordination, the question arises how the conformation of these kind of ligands affects the structure of the corresponding zinc complexes. According to literatures, such complexes can be stable in two ways. In the first way, stabilization is achieved by additional coordination of monodentate Lewis bases in order to create a five-coordinated zinc center [1], [2], [22]. In the second way, the bis(thiosemicarbazon) ligand is fully deprotonated, the obtained dianion acts as a bridging tetradentate ligand coordinated to two Zn(II) ions with a tetrahedral geometry [9], [23]. An example of a bis(thiosemicarbazone) acting as a bridge between two zinc atoms is the complex derived from 2,6-Diacetylpyridine bis(hexamethyleneiminylthiosemicarbazone), in which the anionic thiosemicarbazone moieties lead to a [ZnN2S2] unit for each atom in a binuclear compound [16]. As an extension of this we have now considered the chelating properties of the bis(thiosemicarbazone) schiff base ligands H2L1, H3L2, H2L3, and H3L4 in order to improve our understanding of behavior of these ligand systems and to further investigating of substituent effect, we have synthesized and characterized zinc(II) complexes of the ligands. The analysis of the crystallographic data clearly shows the role of hydroxyl group on the structure adopted by the complexes. It should be noted that to date any type of binuclear alkoxide-bridged structure have not been observed for zinc complexes with a bis(thiosemicarbazone) ligand.
Section snippets
Materials
Bis(thiosemicarbazone) ligands, H2L1, H3L2, H2L3, and H3L4 were prepared according to literature methods [20], [21]. Other reagents and solvents was obtained from Alfa Aesar and Merck and was used as received.
Physical measurements
1H NMR spectra were recorded on a Bruker Avance 400 in DMSO-d6 solvent and chemical shifts are relative to residual solvent protons as internal standard at room temperature. Elemental analyses were performed on Elementar Vario EL III. FTIR spectra were recorded on an FTIR Spectrometer
Synthesis of the complexes
The reaction of zinc(II) acetate with the ligands yielded two types of products with high yields (Scheme 1). Interestingly, the interaction of the zinc(II) with the hydroxyl-containing ligands produced binuclear complexes, even when the ligand to metal molar ratio was equal. This result suggests that the obtained complexes are the thermodynamically products. But its reaction with the non-hydroxyl ligands produced dimer complexes. The presence of bulky groups (PhCl, PhBr) on the terminal
Conclusion
In this paper, we reported the synthesis and spectral studies of new zinc complexes of N(4)-substituted bis(thiosemicarbazone) ligands and showed the influence of different substitutions on the phenyl rings of thiosemicarbazone part and the role of hanging hydroxyl group on the structural and spectral properties of complexes. With using the structural modification on the ligand, we were able to synthesize four zinc(II) complex with two different geometric structures. The interaction of zinc(II)
Acknowledgements
S. H. would like to thank Christian Wichmann, University of Melbourne for his help. S. H. acknowledge the University of Tabriz for financial support.
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